Filter GH webhooks pattern matching

[humitos]

We are using Python's fnmatch to match the files and filter (whether trigger or not the build). This implementation has a few limitations:

1. we cannot match multiple type of files/paths (mymodule/*.py and docs/*.rst
2. it doesn't support ** for nested directories (**/.rst)
3. ... maybe other ones

I think a basic use case we will want to support here would be: .readthedocs.yaml & docs/* & requirements/docs.txt. Currently, we cannot do that and we need to create three different rules for that.

I'm creating this issue to think about how to model this. We may want to make the field a TextField and accept one rule per line, as we've done in other places. I'd say that's probably the easiest way to implement and to communicate to the user as well. This solves 1) but it doesn't solve 2)

[stsewd]

it doesn't support ** for nested directories (**/.rst)

* in fnmatch defaults to match everything, even /, that's what ** does.

[humitos]

OK. It doesn't support just a single * then.